An occupant protection system installed in a vehicle uses an acceleration sensor to detect that the vehicle has been subjected to a collision load from a vehicle front and cause an occupant protection device control unit to control the occupant protection system in accordance with a detection signal from the acceleration sensor. The occupant protection device control unit is a unit that is housed in a casing and installed in the front section of the vehicle body. Some of occupant protection device control units have an acceleration unit integrated thereinto, which are called a sensor integrated type. Integration of sensors allows the simplification of, for example, a signal system of the occupant protection system. Such a casing that houses an occupant protection device control unit is known by, for example, Japanese Unexamined Patent Application Publication No. 2010-083382.
The casing for an occupant protection device control unit known by Japanese Unexamined Patent Application Publication No. 2010-083382 is composed of a resin-made main body having its bottom surface opened and a cover that covers the bottom surface of the main body. A substrate for the occupant protection device control unit is secured to the main body with screws, together with the cover located below the substrate.
The main body has a plurality of brackets formed thereon in an integral manner. Each of the brackets is mounted on a vehicle body. A collision load to which the vehicle body is subjected is transmitted from the vehicle body to the main body through the brackets and to the substrate through the screws and then to an acceleration sensor installed in the substrate.
The inventors found the followings. It is preferable that the collision load is transmitted from the vehicle body to the acceleration sensor as directly as possible without the resin-made main body in a transmission path in order to allow the collision load to be transmitted to the acceleration sensor more quickly and exactly. To that end, it is conceivable that the cover is changed to a metallic base and only the base is installed to the vehicle body. If the metallic base is used, the brackets formed on the main body are not required.
However, the metallic base must bear a great burden since it is fully subjected to a collision load. In addition, the base must protect the occupant protection device control unit housed in the casing from an excessive collision load. Use of, for example, a thicker base would be a solution for ensuring the rigidity of the base, but it would have a disadvantage in reducing weight and cost of the casing.